Effect of Minocycline on Nociceptive Change and Microglial Activation in a Rat Model of Sciatic Neuropathy

Gu, Eun Young; Han, Hyung Soo; Park, Jae Sik

Abstract

Nerve injury leads to chronic neuropathic pain syndromes. Activation of microglia has been studied to investigate the role in pain development. Minocycline is known as a potent inhibitor of microglial activation in many types of the brain injury models. But it is not known whether minocycline interferes with pain and microglial activation after the peripheral nerve injury. In this study, we investigated the time course of pain and microglial activation after sciatic nerve injury and also tested the effect of minocycline using sciatic nerve ligation model. All experiments were performed using 150~180 g male Sprague-Dawley rats. The chronic constriction injury (CCI) of the sciatic nerve with four 4.0 chromic gut suture was used to induce neuropathic pain in the left sciatic nerve. The behavioral response of rats to the stimuli (heat, cold & pressure) was assessed by measuring the lifting of the foot and the avoidance of touching the floor at pre-surgical day 1, post-surgical day 1, 4, 7, and 10. The L4 ~6 spine was fixed and used to detect microglia. Oral minocycline (50 mg/kg) was administered daily to the last day of the experiment. Minocycline was administered to one group of rats from pre-surgical day 1 and minocycline treatment was initiated from post-surgical day 1, 3, and 5 in other groups. Neuropathic pain was evident from day 4 and the peak response was observed at 10 days after CCI. Minocycline significantly attenuated neuropathic pain even when treatment was delayed by 3 days. But, it had no effet when treatment initiated 5 days after injury. Minocycline also attenuated microglial activation. In summary, a correlation was evident between the neuropathic pain and microglial activation in our model and minocycline reduced both development of pain and microglial activation. Thus, minocycline can be a good candidate for the treatment of neuropathic pain. However, the administration should be initiated prior to microglial activation.